But until now the relationship between bird coloring and testosterone had eluded biologists.

Researcher Julio Blas, a biologist at the University of Saskatchewan in Saskatoon, Canada, and colleagues decided to tackle the issue through experiments in Spain with native red-legged partridges (map of Spain).

Blas's team increased the testosterone of male partridges during the mating season and saw a 20 percent rise in carotenoids—which birds get from food such as berries and insects—in their blood and livers.

"A bird in good shape should be colorful and also should sing more," said Blas, whose research appears this week in the journal Proceedings of the National Academy of Sciences. "Until now these lines of evidence have been researched independently of each other. What we did is connect these two lines of research."

Immune System Puzzle

The finding could solve another outstanding puzzle.

High levels of testosterone come with a price, as the hormone usually depresses the immune system, increasing birds' susceptibility to disease.

But recent studies have shown that birds manipulated in the lab to have high testosterone could still have robust immune systems.

The link between testosterone and carotenoids may be the answer, Blas says.
Carotenoids help build vitamins and are strong antioxidants—chemicals that help animals detoxify harmful molecules called free radicals.